Charged Particle Beam Device and Axis Adjustment Method Thereof

ABSTRACT

A charged particle beam device is provided in which axis adjustment as a superimposing lens is facilitated by aligning an axis of an electrostatic lens resulting from a deceleration electric field with an axis of a magnetic field lens. The charged particle beam device includes: an electron source; an objective lens that focuses a probe electron beam from the electron source on a sample; a first beam tube and a second beam tube through each of which the probe electron beam passes; a deceleration electrode arranged between the first beam tube and a sample; a first voltage source that forms a deceleration electric field for the probe electron beam between the first beam tube and the deceleration electrode by applying a first potential to the first beam tube; and a first moving mechanism that moves a position of the first beam tube.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.17/056,993, filed Nov. 19, 2020, which is a 371 of InternationalApplication No PCT/JP2018/019568, filed May 22, 2018, the disclosures ofall of which are expressly incorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a charged particle beam device and anaxis adjustment method thereof.

BACKGROUND ART

A scanning electron microscope (SEM) obtains a two-dimensional image ofa scanning area on a sample surface by detecting signal electronsgenerated when a sample is scanned by being irradiated with a focusedprobe electron beam, and displaying a signal intensity of eachirradiation position in synchronization with a scanning signal of anirradiation electron beam.

In recent years, low-acceleration observation with irradiation energy ofabout 1 keV or less is more and more important for a purpose ofobtaining sample information on an extreme surface while avoidingcharging or damage of the sample caused by electron beam irradiationduring an SEM observation. However, chromatic aberration generallyincreases in a low acceleration region, and it is difficult to obtainhigh resolution. In order to reduce this chromatic aberration, there isknown a deceleration optical system in which the sample is irradiated byaccelerating the probe electron beam to pass through an objective lensat a high speed and decelerating the probe electron beam immediatelybefore the sample.

In a method called a boosting method in the deceleration optical system,a cylindrical electrode for applying a positive voltage is providedalong an inner wall of an inner magnetic path of the objective lens ofthe SEM, and the sample is set to a ground potential. In a method calleda retarding method, an objective lens side of an SEM column is kept atthe ground potential and a negative voltage is applied to the sample. Itis characterized that in any method, a passage area of the probeelectron beam from the objective lens side to an electron source sidehas a higher potential than the sample, and an electric field formed bythis potential difference and decelerating the probe electron beamtoward the sample is used as a lens field. By superimposing anelectrostatic lens resulting from a decelerating electric field and amagnetic field lens resulting from a magnetic field of the objectivelens, aberration can be reduced in the low acceleration region and thehigh resolution can be obtained.

In the deceleration optical system, both the electrostatic lens and themagnetic field lens that are superposed determine an irradiation systemperformance. In particular, in a low acceleration voltage region, aninfluence of the electrostatic lens is large. Therefore, in order toobtain a best performance, it is necessary to guide the probe electronbeam to a lens center of each of the magnetic field lens and theelectrostatic lens that are superimposed. This is because, when theprobe electron beam passes through a portion (off-axis) deviated fromthe lens center, off-axis aberration occurs, which adversely affects aspot formation of the irradiation electron beam.

As an adjustment method for causing the probe electron beam to pass acenter of the magnetic field lens, current center axis adjustment thatis performed by minimizing an image movement amount when an excitationcurrent of the objective lens is periodically changed is often used.Thus, the off-axis aberration of the magnetic field lens can beminimized. In addition, since image movement does not occur during focusadjustment during image acquisition, operability of the focus adjustmentis improved.

On the other hand, as an adjustment method for causing the probeelectron beam to pass a center of the electrostatic lens, voltage centeraxis adjustment that minimizes an image movement amount when an appliedvoltage of electrodes forming an electric field of the electrostaticlens is periodically changed is used. Thus, it is possible to minimizethe chromatic aberration generated in the electrostatic lens.

PTL 1 discloses a configuration in which in an electron beam apparatusincluding the deceleration optical system, an electron beam trajectoryis deflected by using an electromagnetic aligner, and axis adjustmentcan be performed. Specifically, a deflector is arranged between theelectrodes forming the electrostatic lens and the objective lens formingthe magnetic field lens, and the probe electron beam is guided to thecenter of the electrostatic lens.

CITATION LIST Patent Literature

-   PTL 1: JP-A-2000-173519

SUMMARY OF INVENTION Technical Problem

In the charged particle beam device, optical axis adjustment for guidingthe probe electron beam to the center of the electrostatic lens or themagnetic field lens by deflecting the trajectory of the irradiationelectron beam that enters the lens field using the electromagneticaligner is often performed. However, in an optical system such as thedeceleration optical system in which the electrostatic lens and themagnetic field lens are superimposed, there is no guarantee that theaxis of the electrostatic lens and the axis of the magnetic field lensare linearly aligned in advance, and in many cases the axes are notaligned. Therefore, there is a problem that the axis adjustment as asuperimposing lens cannot be sufficiently performed only by deflectingthe electron beam trajectory.

In addition, in the axis adjustment method disclosed in PTL 1, it isnecessary to arrange the deflector between the objective lens formingthe magnetic field lens and the electrodes forming the electrostaticlens. However, it is desirable to arrange the objective lens as close tothe sample as possible in order to reduce a focal length and theaberration. Therefore, when the deflector is arranged between theelectrostatic lens and the magnetic field lens, a mechanical constraintthereof reduces a degree of freedom in designing an objective lensstructure, and as a result, it is difficult to bring the magnetic fieldlens or the electrostatic lens sufficiently close to the sample.

Solution to Problem

In order to solve the above problem, a configuration described in theclaims is adopted. For example, a charged particle beam device accordingto an embodiment includes: an electron source; an objective lens thatfocuses a probe electron beam from the electron source on a sample; afirst beam tube and a second beam tube through each of which the probeelectron beam passes; a deceleration electrode arranged between thesample and the first beam tube arranged closer to an objective lens sidethan the second beam tube; a first voltage source that forms adeceleration electric field for the probe electron beam between thefirst beam tube and the deceleration electrode by applying a firstpotential to the first beam tube; and a moving mechanism that moves aposition of the first beam tube.

Advantageous Effect

A charged particle beam device can be provided in which axis adjustmentas a superimposing lens is facilitated by aligning an axis of anelectrostatic lens resulting from the deceleration electric field withan axis of a magnetic field lens.

Other technical problems and novel characteristics will be apparent froma description of the present description and the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view showing a charged particlebeam device according to a first embodiment.

FIG. 2 is a schematic cross-sectional view showing a charged particlebeam device including a deceleration optical system in the related art.

FIG. 3 is a schematic cross-sectional view showing a modification of thecharged particle beam device according to the first embodiment.

FIG. 4 is a schematic cross-sectional view showing a modification of thecharged particle beam device according to the first embodiment.

FIG. 5 is a schematic cross-sectional view showing a charged particlebeam device according to a second embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the invention will be described withreference to the accompanying drawings. Although the accompanyingdrawings show specific embodiments in accordance with the principles ofthe invention, the embodiments are provided for the purpose ofunderstanding the invention, and are not to be used for limitinginterpretation of the invention. In all the drawings showing theembodiments and the modifications, those having the same function aredesignated by the same reference numerals, and the repeated descriptionthereof will be omitted.

Embodiment 1

FIG. 1 is a schematic cross-sectional view showing a charged particlebeam device according to a first embodiment. An SEM to which a boostingmethod is applied is shown as an example of the charged particle beamdevice. An SEM column 1 includes an electron source 2 including amechanism for irradiating a sample 12 with a probe electron beam, anaperture 5 for limiting a diameter of the probe electron beam,electronic lenses such as a condenser lens 3 or an objective lens 4,deflectors 7 for causing the sample 12 to be scanned with the probeelectron beam, and a detector 6 for detecting signal electrons generatedfrom the sample 12. The objective lens 4 forms a magnetic field lens forfocusing the probe electron beam on the sample 12 by generating amagnetic field using, for example, a coil. The SEM column 1 may includeother components (a lens, an electrode, a detector, or the like) otherthan the above, and is not limited to the above configuration.

Further, the SEM includes a sample chamber 14. The sample chamber 14 isprovided with a sample table 13 on which the sample 12 is mounted. Thesample table 13 includes a mechanism for tilting and moving the sample12, and an observation region of the sample 12 can be determined by thismechanism. In addition, the SEM includes a vacuum exhaust facility (notshown) for vacuum exhausting the sample chamber 14 and the SEM column 1.

The SEM also includes a control unit 16 that controls an entire SEM. Thecontrol unit 16 controls each component of the SEM and executes variousinformation processing. The control unit 16 includes an image displaydevice (not shown), and displays an SEM image generated based oninformation obtained from the detector 6 on the image display device.

The control unit 16 may be implemented by using, for example, ageneral-purpose computer, or may be implemented as a function of aprogram executed on the computer. The computer includes at least aprocessor such as a central processing unit (CPU), a storage unit suchas a memory, and a storage device such as a hard disk. A processing ofthe control unit 16 may be realized by storing a program code in thememory and causing the processor to execute each program code. A part ofthe control unit 16 may be configured with hardware such as a dedicatedcircuit board.

The objective lens 4 of the SEM column 1 is an out-lens type having asmaller leakage magnetic field with respect to the sample 12. Inaddition, the SEM column 1 includes a boosting means as the decelerationoptical system. Specifically, in the SEM column 1, a cylindrical secondbeam tube 9 is provided from the electron source 2 along an opticalaxis, and a first beam tube 8 is provided along an inner wall of anobjective lens magnetic path of the objective lens 4. In addition, avoltage source (a boosting voltage source) 15 for applying a boostingvoltage to the first beam tube 8 and the second beam tube 9 is provided.A voltage from the boosting voltage source 15 is controlled by thecontrol unit 16. In addition, a deceleration electrode 10 is provided ata tip portion of the objective lens. The first beam tube 8 and thesecond beam tube 9 are set to a higher potential than the decelerationelectrode 10 by applying the voltage from the boosting voltage source15. Thus, a decelerating electric field for the probe electron beam isformed between a sample-side end portion of the first beam tube 8 andthe deceleration electrode 10, and the probe electron beam isdecelerated by a lens action when passing through this electric field.

In particular, in order to obtain high resolution under an observationcondition where irradiation energy of the probe electron beam is 5 keVor less, it is necessary to set the first beam tube 8 to a highpotential with respect to the sample 12 and form the decelerationelectric field. For example, a potential difference between the sample12 and the first beam tube 8 is set to about 10 kV. As strength of theleakage electric field from the SEM column 1 increases, an electrostaticlens action having a shorter focal length can be obtained near thesample 12. Thus, aberration is reduced and an effect of improvingresolution is enhanced.

In addition, the deceleration electrode 10 may form a part of theobjective lens 4. The deceleration electrode 10 may be formed of amagnetic material and may be magnetically coupled to the objective lensmagnetic path, or the objective lens magnetic path and the decelerationelectrode 10 may be configured to function as one magnetic circuit.

In order to obtain a best irradiation system performance in the SEM,axis adjustment that properly adjusts a path that passes the lenses ofthe probe electron beam is required. In particular, in the SEM includingthe deceleration optical system as in this embodiment and using themagnetic field lens and the electrostatic lens in a superimposingmanner, when the probe electron beam passes through a portion deviatedfrom a lens center axis of the electronic lenses, the irradiation systemperformance is deteriorated due to an effect of off-axis aberration ofthe lens. Since the irradiation system performance deteriorates in acase where the probe electron beam passes an off-axis portion of any ofthe electrostatic lens and the magnetic field lens, it is necessary toperform the axis adjustment such that axes of both the electrostaticlens and the magnetic field lens are aligned and the probe electron beampasses through the center.

In this embodiment, the axis adjustment of a superimposing lens isperformed by moving an optical axis of the electrostatic lens. Aposition of the optical axis of the electrostatic lens is determined bya positional relationship between the first beam tube 8 and thedeceleration electrode 10. Therefore, by moving a position of the firstbeam tube 8, adjustment of an electrostatic lens axial position can berealized. Therefore, in this configuration, a moving mechanism 11capable of mechanically moving the position of the first beam tube 8that forms the decelerating electric field is provided.

The first beam tube 8 needs to be movable by the moving mechanism 11while maintaining electrical insulation from the objective lens magneticpath or the deceleration electrode 10. A structure may be employed inwhich a gap is provided between the first beam tube 8 and the objectivelens magnetic path or the deceleration electrode 10 and a spacetherebetween is insulated. In addition, an electrically insulatedstructure in which a deformable insulator is interposed between thefirst beam tube 8 and the objective lens magnetic path or thedeceleration electrode 10 may be adopted. With such a structure, it ispossible to move the position of the first beam tube 8 in a state wherea high voltage is applied to the first beam tube 8.

As a comparative example, a schematic view of an SEM column providedwith a deceleration optical system in the related art is shown in FIG. 2. Compared with a configuration of FIG. 1 , a configuration is employedin which a beam tube 17 to which a high potential is applied is notdivided, an accelerating electric field is formed at an end portion onan electron source side and a decelerating electric field is formed atan end portion on a sample side with respect to the probe electron beam,and each electric field has a lens action. When the probe electron beamdoes not pass through a center of an acceleration lens 31, off-axisaberration occurs and adversely affects an irradiation system.Therefore, when the axis adjustment of an optical system is performed,first, an electron beam trajectory is guided such that the probeelectron beam passes through the center of the acceleration lens 31. Forexample, movement of a position of the electron source 2 and trajectorydeflection using an electromagnetic deflector are often performed.

On the other hand, as described above, it is also necessary to adjust anaxis of a deceleration lens 32 that is superimposed with the magneticfield lens and that is formed at a sample-side end portion of the beamtube. Although the beam tube 17 is fixed in the SEM column in therelated art, when it is assumed that the beam tube 17 can be moved andthe beam tube 17 is moved to adjust a center position of thedeceleration lens 32, as a result, not only the center of thedeceleration lens 32 but also the center of the acceleration lens 31moves at the same time. That is, the deceleration lens 32 alone cannotbe adjusted independently, and the axis adjustment of the decelerationlens 32 causes axial misalignment in the acceleration lens 31. Actually,the probe electron beam does not go straight along a line verticallydropped from the electron source 2 toward the sample 12 as shown in FIG.2 , but goes toward the sample while a traveling direction is bent bythe deflector or lens provided in the SEM column. Therefore, an incidentangle of the probe electron beam to the acceleration lens 31 and anincident angle to the deceleration lens 32 are usually different fromeach other, and it is not realistic that axial positions of theacceleration lens 31 and the deceleration lens 32 can be optimized atthe same time by position adjustment of one beam tube 17.

It is one of characteristics in the device of the first embodiment shownin FIG. 1 , that the beam tube to which the high potential is applied isprovided and divided into two, the first beam tube 8 on the sample sideand the second beam tube 9 on the electron source side. Due to thedivision, the position of the second beam tube 9 does not change evenwhen the first beam tube 8 is moved. Therefore, the position of thedeceleration lens formed at the sample-side end portion of the firstbeam tube 8 can be adjusted without changing the position of theacceleration lens formed at the electron source-side end portion of thesecond beam tube 9. In other words, it is possible to prevent anoccurrence of the axial misalignment of the acceleration lens during theposition adjustment of the deceleration lens.

At least during the axis adjustment, it is necessary to keep the firstbeam tube 8 and the second beam tube 9 at the same potential. When apotential of the first beam tube 8 and a potential of the second beamtube 9 are different, an electric field having a lens action occursbetween two beam tubes. Due to movement of the first beam tube 8 in thissituation, an axial misalignment of the electrostatic lens between thefirst beam tube 8 and the second beam tube 9 occurs. In order to avoidthe axial misalignment, the potentials of the first beam tube 8 and thesecond beam tube 9 are set to the same potential during the axisadjustment. Since no electric field is formed between beam tubes of thesame potential, no lens action occurs. Therefore, the axial adjustmentof only the deceleration lens formed between the first beam tube 8 andthe deceleration electrode 10 can be performed by moving the position ofthe first beam tube 8.

As will be described later, it is desirable to perform the axisadjustment by moving the position of the first beam tube 8 whilevisually recognizing the image. Therefore, it is desirable that themoving mechanism 11 of the first beam tube 8 is a mechanism that canadjust the first beam tube 8 from an outside of the SEM column 1. Thus,it is easier to perform the axis adjustment while visually recognizingthe image. The moving mechanism 11 may be, for example, a mechanism thatmoves the first beam tube 8 by pushing a rod-shaped instrument such as ascrew through an elongated hole formed in an outer wall of the columnlocated outside the first beam tube 8. In addition, some of the screwsmay have spring properties, or the position adjustment may be performedby using a dedicated jig.

In this configuration, the axis of the electrostatic lens is alignedwith the axis of the magnetic field lens by moving the first beam tube8. Therefore, even when a trajectory of the probe electron beam changesdue to changes in an accelerating voltage or a probe current, the axesof the electrostatic lens and the magnetic field lens are kept aligned.Therefore, it is possible to obtain a best performance by deflecting theprobe electron beam trajectory by the deflector and guiding thetrajectory to an aligned axis of the electrostatic lens and the magneticfield lens. The deflector that changes the probe electron beamtrajectory may be a magnetic field type using a coil or an electrostatictype using a pair of electrodes.

An axis adjustment procedure in this embodiment will be described.

(1) At first, the probe electron beam trajectory is aligned with acurrent center axis of the objective lens 4 (the magnetic field lens).First, the objective lens 4 is driven to focus the probe electron beamon the sample 12. When an excitation current of an objective lens coilis changed periodically, the image moves in synchronization with thechange of the excitation current. A position of the electron source 2 orthe deflector 7 is adjusted to change the probe electron beam trajectoryin order to minimize a movement of the image. At this stage, lensesother than the objective lens are not operated, and the potentials ofthe first beam tube 8 and the second beam tube 9 are also set to areference potential (GND).

Next, the condenser lens 3 is set to a predetermined excitation amount,the excitation current supplied to the objective lens 4 is periodicallychanged again, and a position of the condenser lens 3 or the deflector 7is adjusted to change the probe electron beam trajectory in order tominimize the movement of the image. The condenser lens 3 has a functionof adjusting an aperture amount to obtain a desired irradiation currentamount. It is acceptable as long as the excitation amount set at thistime is set so as to have appropriate optical conditions according to amaterial to be observed, for example.

As described above, since the probe electron beam passes through thecurrent center axis of the objective lens 4, a movable aperture 5 isfinally inserted such that the probe electron beam passes through acenter of the movable aperture 5. Again, the excitation current suppliedto the objective lens 4 is periodically changed and a position of themovable aperture 5 is changed to minimize the movement of the image. Asdescribed above, the probe electron beam is adjusted to pass through thecurrent center axis of the magnetic field lens formed by the objectivelens 4.

(2) At the second, the optical axis of the deceleration lens (theelectrostatic lens) is aligned with the current center axis of theobjective lens 4 (the magnetic field lens). A predetermined voltage isapplied to the first beam tube 8 and the second beam tube 9. Theobjective lens 4 and the condenser lens 3 are driven, and the probeelectron beam is focused on the sample in a state of the aperture 5 isinserted. The excitation current supplied to the objective lens 4 isperiodically changed and the position of the first beam tube 8 isadjusted by the moving mechanism 11 in order to minimize the movement ofthe image. Thus, the current center axis of the objective lens 4 (themagnetic field lens) and the optical axis of the deceleration lens (theelectrostatic lens) are aligned with each other.

(3) In addition to the above adjustment, it is desirable to performadjustment of aligning the current center axis of the objective lens 4(the magnetic field lens) with a voltage center axis of the decelerationlens (the electrostatic lens). In this case, (3-1) the aperture 5 ismoved or the deflector 7 is adjusted to change the probe electron beamtrajectory in order to minimize the movement of the image caused byperiodically changing the voltage applied to the first beam tube 8.(3-2) Next, the position of the first beam tube 8 is adjusted in orderto minimize the movement of the image caused by periodically changingthe excitation current supplied to the objective lens 4. (3-3)Procedures of the adjustment (3-1) and the adjustment (3-2) areperformed repeatedly.

By the above adjustment (3), the current center axis of the magneticfield lens and the voltage center axis of the electrostatic lens arealigned, and the probe electron beam is adjusted to pass through thecurrent center axis of the magnetic field lens and the voltage centeraxis of the electrostatic lens that are aligned.

FIG. 3 shows a modification in which the first beam tube 8 and thesecond beam tube 9 are provided with different voltage sources in thecharged particle beam device according to the present embodiment. Afirst voltage source 15 a is connected to the first beam tube 8 and asecond voltage source 15 b is connected to the second beam tube 9. Thepotential applied by each voltage source is controlled by the controlunit 16. The control unit 16 includes a mode in which the voltagesources 15 a and 15 b are controlled such that the potentials of thefirst beam tube 8 and the second beam tube 9 are the same potential.

In an axis adjustment procedure in the present modification, when theadjustment for aligning the axis of the deceleration lens formed betweenthe first beam tube 8 and the deceleration electrode 10 with the axis ofthe objective lens (the magnetic field lens) is performed, the mode inwhich the potentials of the first beam tube 8 and the second beam tube 9are set to the same potential is set. In the same potential mode, bymoving the second beam tube 9 using the moving mechanism 11, theposition of the deceleration lens can be adjusted alone.

In the present modification, the potentials of the first beam tube 8 andthe second beam tube 9 are not limited to the same potential exceptduring the adjustment, and can be set to any potential. Thus, it ispossible to set an optimum potential for the irradiation system and adetection system according to requirements of an entire optical system.For example, when electron beam analysis is performed using a scanningelectron microscope, a condition of setting the second beam tube 9 to ahigher potential than the first beam tube 8 may be set for a purpose ofimproving electron source brightness and increasing the probe current.When the potentials of the first beam tube 8 and the second beam tube 9are set to different potentials, the electrostatic lens is formedbetween the first beam tube 8 and the second beam tube 9, but an effectdue to this formation can be compensated by using the deflector 7, forexample.

Further, FIG. 4 shows a modification in which the beam tube is dividedinto three or more in the charged particle beam device of the presentembodiment. For example, the beam tube is divided into three tubes, asample-side beam tube (the first beam tube) 8, an intermediate beam tube(a third beam tube) 18, and an electron source-side beam tube (thesecond beam tube) 9, and moving mechanisms 11 a to 11 c and voltagesources 15 a to 15 c are provided for the respective beam tubes. In anexample of FIG. 4 , an example in which the moving mechanism 11 is alsoprovided in the second beam tube 9 is shown, but this example is alsoapplicable to other configuration examples such as FIG. 1 .

The present modification is effective when it is necessary to set apotential of the path of the probe electron beam into a plurality ofstages depending on the requirements of the irradiation system or thedetection system. For example, a configuration is conceivable in whichthe intermediate beam tube 18 is set to a ground potential, and thesample-side beam tube 8 and the electron source-side beam tube 9 are setto the higher potential than the sample 12. In this case, a degree offreedom in designing the detection system is improved by providing aground potential portion while obtaining an effect of improving theelectron source brightness and an effect of reducing the aberration byboosting. A detector 6′ provided at the ground potential may be, forexample, a detector using a deflection field.

In the modification shown in FIG. 4 , when the axis adjustment of theelectrostatic lens is performed, the potentials given to the respectivebeam tubes are controlled such that single electrostatic lens adjustmentis realized. For example, when the axis adjustment of the electrostaticlens formed between the sample-side beam tube 8 and the decelerationelectrode 10 is performed, it is acceptable as long as the intermediatebeam tube 18 and the sample-side beam tube 8 are set to the samepotential, and the position of the sample-side beam tube 8 is moved.Similarly, for example, when the axis adjustment of the electrostaticlens formed between the intermediate beam tube 18 and the sample-sidebeam tube 8 is performed, it is acceptable as long as the electronsource-side beam tube 9 and the intermediate beam tube 18 are set to thesame potential, and the position of the intermediate beam tube 18 ismoved. The potential applied by each voltage source is controlled by thecontrol unit 16. The control unit 16 includes a mode in which thevoltage sources 15 a and 15 c are controlled such that the potentials ofthe sample-side beam tube 8 and the intermediate beam tube 18 are set tothe same potential, and a mode in which the voltage sources 15 b and 15c are controlled such that the potentials of the electron source-sidebeam tube 9 and the intermediate beam tube 18 are set to the samepotential. Thus, a configuration is obtained in which the axis of theelectrostatic lens formed at an end portion of each beam tube can beadjusted using the moving mechanism 11.

The position at which the beam tube is divided is not limited unlessotherwise there is a requirement from the irradiation system or thedetection system. However, since a purpose of the beam tube is tooriginally keep an inside of the beam tube in a stable potential state,it is necessary to prevent the electric field from entering through thegap between divided beam tubes as much as possible. Therefore, in thepresent embodiment, both beam tubes are provided with flange portions atan end portion where the beam tubes are in contact with each other. Forexample, in the example of FIG. 1 , a disk-shaped flange portionprotruding from the end portion of the first beam tube 8 and adisk-shaped flange portion protruding from the end portion of the secondbeam tube 9 face each other, and thus an unintended electric field isprevented from entering through the gap between the first beam tube 8and the second beam tube 9.

Embodiment 2

As a second embodiment, an SEM to which the retarding method is appliedas the deceleration optical system will be described. There is noessential difference between a boosting optical system and a retardingoptical system in that the decelerating electric field formed by thepotential difference between the sample and a passage area of the probeelectron beam on the electron source side from the objective lens. Thatis, the invention, which is effective in the boosting optical system,can also obtain the same effect in the retarding optical system.

FIG. 5 is a schematic configuration diagram showing a charged particlebeam device according to the second embodiment. The sample table 13includes a retarding voltage source 21 that can apply a voltage to thesample 12. The voltage from the voltage source 21 may be controlled bythe control unit 16. Typically, the SEM column 1 and the first beam tube8 and the second beam tube 9 that are in the column are set to theground potential, and a negative voltage is applied to the sample 12.Thus, the decelerating electric field for the probe electron beam occursbetween the sample-side end portion of the first beam tube 8 and thesample 12, and the resolution of the SEM can be improved. It isdesirable that the negative voltage applied to the sample 12 forimproving the resolution is set such that the potential differencebetween the sample 12 and the first beam tube 8 is 1 kV or more, and thelarger the potential difference, the greater a resolution improvingeffect. However, a suitable voltage value is not limited thereto sincethe voltage value may change depending on a working distance (WD)between the sample 12 and the tip portion of the objective lens 4.

In the present embodiment, the position of the deceleration lens to besuperimposed on the magnetic field lens is determined by a positionalrelationship among the first beam tube 8, the tip portion of theobjective lens and the sample 12. Therefore, the axis adjustment of thedeceleration lens can be realized by moving the position of the firstbeam tube 8. In addition, similarly to the first embodiment, by settingthe first beam tube 8 and the second beam tube 9 to the same potentialduring the adjustment, the axis adjustment can be performed for thedeceleration lens alone. Therefore, by aligning a center axis of thedeceleration lens with a center axis of the magnetic field lens andguiding the probe electron beam to a center axis of the superimposinglens, it is possible to reduce the off-axis aberrations and maximize theirradiation system performance.

The axis adjustment in the present embodiment can be performed, forexample, by the procedure the same as in the first embodiment.

(1) At first, the probe electron beam trajectory is aligned with thecurrent center axis of the objective lens 4 (the magnetic field lens).First, the objective lens 4 is driven to focus the probe electron beamon the sample 12. In this state, the excitation current of the objectivelens coil is periodically changed, and the position of the electronsource 2 or the deflector 7 is adjusted to change the probe electronbeam trajectory in order to minimize the movement of the image.

Next, the condenser lens 3 is set to predetermined excitation, theexcitation current supplied to the objective lens 4 is periodicallychanged again, and the position of the condenser lens 3 or the deflector7 is adjusted to change the probe electron beam trajectory in order tominimize the movement of the image. Finally, the movable aperture 5 isinserted on the optical axis, and again, the excitation current suppliedto the objective lens 4 is periodically changed and the position of themovable aperture 5 is adjusted in order to minimize the movement of theimage.

(2) Second, the optical axis of the deceleration lens (the electrostaticlens) is aligned with the current center axis of the objective lens 4(the magnetic field lens). The objective lens 4 and the condenser lens 3are driven, the negative voltage is applied to the sample 12 and theprobe electron beam is focused on the sample in a state where theaperture 5 is inserted. The excitation current of the objective lens 4is periodically changed and the position of the first beam tube 8 ischanged by the moving mechanism 11 in order to minimize the movement ofthe image. Thus, the current center axis of the objective lens 4 (themagnetic field lens) and the optical axis of the deceleration lens (theelectrostatic lens) can be aligned with each other.

(3) In addition to the above adjustment, it is desirable to performadjustment of aligning the current center axis of the objective lens 4(the magnetic field lens) with a voltage center axis of the decelerationlens (the electrostatic lens). In this case, (3-1) the aperture 5 ismoved or the deflector 7 is adjusted to change the probe electron beamtrajectory in order to minimize the movement of the image caused byperiodically changing a retarding voltage. (3-2) Next, the position ofthe first beam tube 8 is adjusted in order to minimize the movement ofthe image caused by periodically changing the excitation currentsupplied to the objective lens 4. (3-3) Procedures of the adjustment(3-1) and the adjustment (3-2) are performed repeatedly.

By the above adjustment (3), the current center axis of the magneticfield lens and the voltage center axis of the electrostatic lens arealigned, and the probe electron beam is adjusted to pass through thecurrent center axis of the magnetic field lens and the voltage centeraxis of the electrostatic lens that are aligned.

As a modification of the present embodiment, a configuration may beemployed in which a positive voltage is applied to the first beam tube 8and the second beam tube 9 and the boosting method and the retardingmethod are used together. By applying the positive voltage having apolarity opposite to that of the retarding voltage to the first beamtube 8, the potential difference between the first beam tube 8 and thesample 12 is further increased, and further improvement of theirradiation system performance can be expected. In this case, theposition of the deceleration lens to be superimposed on the magneticfield lens is also determined by the positional relationship among thefirst beam tube 8, the tip portion of the objective lens and the sample12. Therefore, it is possible to realize the adjustment for aligning theaxis of the electrostatic lens with the axis of the magnetic field lensby moving the position of the first beam tube 8.

On the other hand, a configuration may be employed in which when ageneral retarding method is applied and the voltage applied to the beamtube is set to the ground voltage, the first beam tube 8 in FIG. 5 isreplaced with an electrode for generating the decelerating electricfield between the electrode and the sample, and the second beam tube 9is removed. In this case, the position of the deceleration lens to besuperimposed on the magnetic field lens is also determined by thepositional relationship among the electrode 8, the tip portion of theobjective lens and the sample 12. Therefore, it is possible to realizethe adjustment for aligning the axis of the electrostatic lens with theaxis of the magnetic field lens by moving the position of the electrode8. The electrode 8 can be a cylindrical electrode including an openingthrough which the probe electron beam passes or a disc electrode.

The invention is not limited to the above embodiments and includesvarious modifications. The embodiments described above have beendescribed in detail for easy understanding of the invention, and are notnecessarily limited to those including all the configurations describedabove. In addition, partial configuration of one embodiment may bereplaced with the configuration of another embodiment. In addition, theconfiguration of another embodiment can be added to the configuration ofone embodiment. In addition, with respect to partial configuration ofeach embodiment, other configurations can be added, deleted, orreplaced. For example, although the SEM is described as an embodiment,the invention is applicable to other charged particle beam devices andalso to a composite charged particle beam device in which a plurality ofcharged particle beam devices are combined.

REFERENCE SIGN LIST

-   -   1 SEM column    -   2 electron source    -   3 condenser lens    -   4 objective lens    -   5 aperture    -   6, 6′ detector    -   7 deflector    -   8 first beam tube (electrode)    -   9 second beam tube    -   10 deceleration electrode    -   11 moving mechanism    -   12 sample    -   13 sample table    -   14 sample chamber    -   15 boosting voltage source    -   16 control unit    -   17 beam tube    -   18 intermediate beam tube    -   21 retarding voltage source

1. A charged particle beam device, comprising: an electron source; anobjective lens that focuses a probe electron beam from the electronsource on a sample; an electrode having an opening through which theprobe electron beam passes; a voltage source that forms a decelerationelectric field for the probe electron beam between the electrode and thesample by applying a first potential to the sample; a moving mechanismthat moves a position of the electrode; a beam tube that is arrangedcloser to an electron source side with respect to the electrode andthrough which the probe electron beam passes; and a second voltagesource that applies a second potential having a polarity opposite to thefirst potential to the electrode and the beam tube.
 2. An axisadjustment method of a charged particle beam device including adeceleration optical system in which a magnetic field lens that focusesa probe electron beam on a sample and an electrostatic lens thatdecelerates the probe electron beam are superimposed, the axisadjustment method comprising: adjusting a probe electron beam trajectoryto pass through a current center axis of the magnetic field lens; andmoving a position of an electrode that forms an electric field of theelectrostatic lens in order to minimize a movement of an image when anexcitation current supplied to the magnetic field lens is periodicallychanged.
 3. The axis adjustment method according to claim 2, comprising:a first step of adjusting the probe electron beam trajectory in order tominimize the movement of the image when a magnitude of the electricfield of the electrostatic lens is periodically changed; and a secondstep of moving the position of the electrode that forms the electricfield of the electrostatic lens in order to minimize the movement of theimage when the excitation current supplied to the magnetic field lens isperiodically changed, wherein the first step and the second step areperformed repeatedly.
 4. The axis adjustment method according to claim2, wherein the charged particle beam device is a charged particle beamdevice to which a boosting method or a retarding method is applied.